Journal of Clinical Monitoring and Computing

, Volume 27, Issue 6, pp 633–637 | Cite as

Quantification of the proportion of motor neurons recruited by transcranial electrical stimulation during intraoperative motor evoked potential monitoring

  • Shunji Tsutsui
  • Hiroshi Yamada
  • Hiroshi Hashizume
  • Akihito Minamide
  • Yukihiro Nakagawa
  • Hiroshi Iwasaki
  • Munehito Yoshida
Original Research


Transcranial motor evoked potentials (TcMEPs) are widely used to monitor motor function during spinal surgery. However, they are much smaller and more variable in amplitude than responses evoked by maximal peripheral nerve stimulation, suggesting that a limited number of spinal motor neurons to the target muscle are excited by transcranial stimulation. The aim of this study was to quantify the proportion of motor neurons recruited during TcMEP monitoring under general anesthesia. In twenty patients who underwent thoracic and/or lumbar spinal surgery with TcMEP monitoring, the triple stimulation technique (TST) was applied to the unilateral upper arm intraoperatively. Total intravenous anesthesia was employed. Trains of four stimuli were delivered with maximal intensity and an inter-pulse interval of 1.5 ms. TST responses were recorded from the abductor digiti minimi muscle, and the negative peak amplitude and area were measured and compared between the TST test (two collisions between transcranial and proximal and distal peripheral stimulation) and control response (two collisions between two proximal and one distal peripheral stimulation). The highest degree of superimposition of the TST test and control responses was chosen from several trials per patient. The average ratios (test:control) were 17.1 % (range 1.8–38 %) for the amplitudes and 21.6 % (range 2.9–40 %) for the areas. The activity of approximately 80 % of the motor units to the target muscle cannot be detected by TcMEP monitoring. Therefore, changes in evoked potentials must be interpreted cautiously when assessing segmental motor function with TcMEP monitoring.


Intraoperative neurophysiological monitoring Transcranial motor evoked potential Triple stimulation technique Recruited motor neurons Spinal surgery 


Conflict of interest

No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Ethical standard

The experiments comply with the current laws of Japan.


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Copyright information

© Springer Science+Business Media New York 2013

Authors and Affiliations

  • Shunji Tsutsui
    • 1
  • Hiroshi Yamada
    • 1
  • Hiroshi Hashizume
    • 1
  • Akihito Minamide
    • 1
  • Yukihiro Nakagawa
    • 1
  • Hiroshi Iwasaki
    • 1
  • Munehito Yoshida
    • 1
  1. 1.Wakayama Medical UniversityWakayamaJapan

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